The present invention relates to the display of data derived from an eddy current detector, particularly in connection with the testing of metal parts, such as steam generator tubes.
Known eddy current detectors include two coils mounted in adjacent arms of a bridge. An alternating current is passed through the bridge to generate an electromagnetic field. This field will be influenced by an adjacent metal body to vary the impedance of one or both coils, thereby creating an unbalance voltage in the bridge, which voltage is the difference between the voltages across the two coils. Thus, the waveform of the unbalance voltage is constituted by a modulated carrier wave at the alternating current frequency.
Eddy current testing is based on an analysis of the phase displacement between the applied alternating current and the unbalance bridge voltage. This can be achieved by obtaining representations of the unbalance voltage components which are in phase with and in quadrature, or 90.degree. out of phase, with the alternating current applied to the bridge.
It is known to display eddy current data on a display monitor in the form of monochrome strip chart waveforms of the two unbalanced voltage components. Such a display may be scrolled until an area of the display containing information of interest is lined up with a selected window. The stored information associated with that display area can be processed to produce a corresponding Lissajous figure representing the relationship between the quadrature and in-phase components associated with the display portion associated with the area of interest.